The metric for evaluating energy performance holds significant importance in the design and operation of air distribution systems aimed at energy efficiency. Heat Removal Efficiency (HRE) stands as a commonly adopted index in this context, with the Energy Utilization Coefficient (EUC) and Effectiveness of Heat Removal (EHR) serving as alternative options. This chapter investigation highlights the inadequacies of these existing indices, demonstrating that they are both unreasonable and incompetent when functioning as energy performance indicators. Additionally, this chapter puts forward a novel index termed Thermal Utilization Effectiveness (TUE), whose efficacy as an energy performance metric is validated through theoretical examinations and experimental tests conducted on stratum ventilation, displacement ventilation, and underfloor air distribution systems. The EUC is deemed unsuitable as an energy performance index because it fails to take into account the relative impacts that the occupied and unoccupied zones have on the overall energy performance of air distribution. In the case of EHR, its inability to qualitatively differentiate the energy performance of various air distribution systems stems from the fact that it relies on a benchmark point that is not fixed but rather floats or varies under different conditions. HRE, on the other hand, encounters challenges in quantitatively distinguishing energy performance due to its unequal weighting of the thermal energy associated with the air temperature in the occupied zone. In contrast, TUE effectively addresses these shortcomings: it accounts for the relative contributions of the occupied and unoccupied zones by incorporating the exit air temperature, enables qualitative differentiation through the use of two stable benchmark points, and ensures quantitative distinction by applying equal weighting to the thermal energy of the air temperature in the occupied zone. As a result, TUE overcomes the limitations inherent in EUC, EHR, and HRE and is a rational and capable energy performance index for air distribution systems.